As a method for detecting over-rotation due to a slip of a motor-driven wheel in an electrically assisted vehicle or the like to prevent a motor from overdriving, following methods are known. (1) An equivalent converted speed of a pedal-driven wheel and a speed of the motor-driven wheel are calculated and compared based on pedal rotation pulses and motor rotation pulses, and when the speed of the motor-driven wheel is too fast, it is determined that the slip of the motor-driven wheel or over-rotation occurred and the motor driving is suppressed. (2) An acceleration of the motor-driven wheel is calculated from the number of rotations of the motor, and when the acceleration is greater than the upper limit that is normally probable, it is determined that the slip of the motor-driven wheel or over-rotation occurred and the motor driving is suppressed.
However, because, in the comparison of the speeds like the method (1), errors of the detection accuracy for the speeds, an error of the wheel radius and margins that include a margin for a handlebar steering angle are considered, it is possible to suppress the motor speed to the extent of the margins when the wheel is racing due to the slip, however it is difficult to completely stop the motor.
More specifically, because, in the comparison of the speeds like the method (1), various kinds of error factors have to be considered in order not to cause any trouble due to the error detection, a quite large fixed-rate speed margin or fixed-quantity speed margin is set. The error factors include a margin for unknown gear shift, a difference between front and rear wheel rotations due to the handlebar steering angle (especially, when a rider makes a U turn on a narrow road or the like, the handlebar is largely turned. Therefore, the difference in the number of rotations between the front wheel and the rear wheel becomes large, and if the rear wheel is the pedal-driven wheel and the front wheel is the motor-driven wheel, dozens of percents or more are set as the margin for the handlebar steering angle.). Furthermore, the error factors also include an error in the tire radius, a tire elastic slip (which is caused by the deformation of the rubber due to the driving force, and is not the slip.), errors for the detection accuracy of both speeds, a quantization error in the detection or calculation, and the like.
Therefore, when the rotation speed of the motor (specifically, the motor-driven wheel) exceeds an originally permissible speed, it may be impossible to suppress the motor rotation. In a notable example, the motor rotation may be permitted to the extent of the speed margin, even when the pedal rotation is zero.
In addition, when the motor-driven wheel is not racing, the difference between the speeds is naturally within the margin. Therefore, even when the pedal does not rotate, however, a huge torque is outputted, it is impossible to suppress the torque.
Even in the method (2), when the load is not so light and the motor gradually performs the over-driving, or when the over-rotation has occurred and the wheel-rotation speed becomes fast, however, the large acceleration is not detected, it is difficult to detect the over-driving.
Thus, there are a lot of cases where the assist driving cannot be suppressed even when the assist driving should be suppressed, and if the motor-driving is not stopped even when the motor-driving should be stopped, a large torque may be outputted, even though the number of rotations is less.
Especially, when there is any trouble in a torque sensor or relating to the torque sensor (e.g. when the torque is applied to the pedal while locking the rear wheel by the key, when the torque sensor gets broken, when clogging due to some extraneous material or mud occurs in the torque sensor, or the like), a situation may occur where the over-rotation of the motor-driven wheel is permitted while recognizing the large torque input in error.
Patent Document 1: Japanese Laid-open Patent Publication No. 2004-243921
Namely, there is no technique for effectively suppressing the over-driving of the motor.